Geology of Mount Vernon Butte
Posted by: 
Volcanoguy
N 44° 24.978 W 119° 05.209
11T E 333864 N 4920229
One of a series of geologic history signs placed by Grant County in the 1970’s.
Waymark Code: WM2Q2H
Location: Oregon, United States
Date Posted: 12/04/2007
Views: 37
Sign about the Geology of Mount Vernon Butte along U.S. Hwy 26 at Clyde Holliday State Park just east of Mount Vernon.
Marker Name: Geology of Mount Vernon Butte
Marker Text: The southern slopes of Mount Vernon Butte, below the skyline, are eroded on Picture Gorge basalt flows which slope southwestward, toward the valley and to your left. Rudely bedded volcanic breccias of the Clarno Formation, which form the low hill in the foreground, actually slope northwestward toward the butte, although they may appear to slope westward. As shown in the cross section, and as you could see from north side, the volcanic breccia also makes up the lower part of Mount Vernon Butte, under the sloping basalt flows.
The differences in directions and angles of dip (slope) of the basalts in the Butte and the breccias in the low hill are caused by movements along a fault or break which crosses the lower slope from left to right about where the juniper trees then out, and goes behind the low hill. The fault can be seen best in afternoon light in the gulch to the right, just south of some brick-red layers in the basalt. The fault movement raised the rocks on the south side probably at least 250 feet and tilted them northward, while those to the north were tilted southward.
A much larger fault, known as the John Day fault, is believed to be buried under the river gravel just south of the rest area. It has been traced nearly 50 miles from east to west along the northern foot of the Strawberry Range southeast of John Day and along the John Day River valley nearly to Dayville. Movement on the John Day fault near here has raised the older rocks south of it at least 1000 feet, so the two faults together have stair-stepped the rocks. The principal movements took place 8-10 million years ago, long before the John Day River had eroded its present wide valley.
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